Blackstone Group LP is about to unleash a seasoned turnaround specialist on some of the private-equity giant's 77 companies.

Blackstone plans to announce Thursday that David Calhoun, now the chief executive of television-ratings firm Nielsen Holdings NV, will join Blackstone as head of its portfolio-operations group, according to him and officials at the buyout firm. The 56-year-old Mr. Calhoun will advise Blackstone-owned companies...